Flocculation and the loss of sediment from the Po River plume

In October 2000, a 100-year flood event in the Po River resulted in the formation of a fine-grained sediment deposit extending up to 10 km from the river mouth. Soon after this event, and for a subsequent period of 2 years, box cores were collected on a grid of stations off of the Po Delta to observe the evolution of the flood deposit. Using a process-based parameterization of the disaggregated inorganic grain size distribution, the evolution of the surficial sediment on the Po shelf since the 2000 flood has been interpreted in the context of particle flocculation dynamics. This method produces estimates of floc limit, the diameter at which the flux of single grains to the seabed equals the flux of flocs, and floc mass fraction, the amount of material deposited as flocs. Floc limit depends on the extent of flocculation in suspension, and floc mass fraction describes the extent of flocculation in the sediment. Immediately after the flood, these two parameters were high at stations located beneath the path of the flood plume as observed in satellite images. The occurrence of a highly flocculated deposit below the path of the plume leads to two hypotheses: (1) the high sediment concentration in the river flood plume produced extensive sediment flocculation in the plume and (2) post-depositional remobilization of sediment delivered from the plume to the seabed was limited in the relatively low-energy environment of the Po prodelta. Floc limit and floc mass fraction estimated from bottom sediment sampling 3 and 10 months after the initial sampling were lower, indicating that during normal discharge, flocculated fine-grained sediment from the Po River settles close to the mouth, leaving only a small amount of material in suspension in the plume for direct deposition onto the prodelta. These findings are consistent with laboratory studies of suspended sediment that show that sediment concentration and turbulent energy exert dominant control on the extent of flocculation and the loss of sediment from suspension.